A major problem of the coating and finishing industry, both in terms of raw material usage and environmental effects, concerns the solvent components of paint. In a spray coating application of a resinous material, the resinous material is typically dissolved in an organic solvent provided with viscosity suitable for spraying. This is required because it has been found that at each stage of the process for atomizing and conveying a resinous material in liquid form to a substrate, the liquid resists high speed deformation. Organic solvents are added to the resinous liquid because they have the effect of separating the molecules of resinous material and facilitating their relative movement making the solution more deformable at high speeds and therefore more susceptible to atomization. Substantial effort has been expended to reduce the volume of liquid solvent components in preparing high solids coating compositions containing about 50% by volume of polymeric and pigmentary solids. Nevertheless, most high solids coating compositions still contain from 15 to 40% by volume of liquid solvent components.
The problem with such a high volume content of liquid solvents in coating compositions is that during handling, atomization or deposition of the coating compositions, the solvents escape and can become air contaminants if not properly trapped. Once the coating composition is applied to a substrate, its solvents escape from the film by evaporation and such evaporated solvents can also contaminate the surrounding atmosphere. Additionally, since most solvents react with oxidants, pollution problems of toxicity, odor and smog may be created. Attempts at overcoming such environmental problems have proven to be costly and relatively inefficient.
It has previously been proposed in Cobbs U.S. Pat. No. 4,247,581 to reduce solvent content in paint by mixing a liquid or gas blowing agent into the paint to produce an easily atomized foamed solution just prior to the spray orifice. Rehman et al U.S. Pat. No. 4,630,774 improved on this concept by designing a foaming chamber and turbulence inducing device into the gun to better control the formation of the foam prior to the spray orifice. U.S. Pat. Nos. 4,505,406; 4,505,957; and, 4,527,712 also disclose concepts for intermixing liquid or gas blowing agents into paint formulations to reduce solvents.
More recently, U.S. Pat. No. 4,923,720 to Lee et al disclosed a method and apparatus for the production of a coating formulation in which a substantial amount of the liquid solvent component is removed and replaced with a supercritical fluid such as supercritical carbon dioxide which functions as a diluent to enhance the application properties of the coating formulation. The supercritical carbon dioxide and some liquid solvent material, e.g., about two-thirds less than is required in other coating compositions, are intermixed with polymeric and pigmentary solids to form a coating material solution or formulation having a viscosity which facilitates atomization through an airless coating dispenser. As the coating material formulation is discharged from the dispensing devices toward a substrate, the supercritical carbon dioxide "flashes off" or vaporizes to assist in atomization of the high solids coating composition and to reduce drying time of the composition on the substrate. Such coating material formulation like the earlier prior art has the advantage of substantially reducing the adverse environmental effects caused by coating compositions having a high solvent content.
It has been observed that in order to produce a coating material solution or formulation with the desired application characteristics, the relative proportion of the liquid coating composition and supercritical carbon dioxide should be maintained at a predetermined ratio or within a predetermined range. The predetermined ratio or range will produce a formulation which is either "single phase" or "multiple phase". The formulation is single phase when the supercritical fluid is dissolved or dispersed within the liquid coating composition to form a single continuous phase of material having a given composition and density. The formulation is multiphase when two or more phases of material are present in the formulation. Where the multiphase formulation has two phases, for example, each phase will have a different composition and density. Normally a single phase formulation will become a two phase formulation as more supercritical carbon dioxide is added into the formulation. Very often the first phase will be a continuous phase and the second phase will be a dispersed phase, e.g., a phase dispersed as bubbles in the first phase, although a continuous second phase would also be possible.
The Lee et al U.S. Pat. No. 4,923,720 discloses an apparatus in which a liquid coating composition and a supercritical fluid are supplied from separate sources to a mixer wherein the two components are combined to form a coating material solution or formulation which is delivered to one or more coating dispensers for deposition onto a substrate. In the embodiment of the system disclosed in the Lee et al patent, the liquid coating composition and supercritical fluid are each introduced into the system by a separate piston pump. These two piston pumps are slaved together by a shaft which extends between the pistons of the two pumps, and the shaft position is adjusted to control the length of the piston stroke of each pump. The length of each piston stroke, in turn, governs the volume of the liquid coating composition and the volume of supercritical fluid entering the system.
One problem with the pumping unit employed in the Lee et al U.S. Pat. No. 4,923,720 is that control of the relative proportion of liquid coating composition and supercritical fluid is difficult. Adjustment of the volume of one material entering the systems automatically produces a change or adjustment in the volume of the other material. This is because the two piston pumps are slaved together by a shaft which is connected between the pistons thereof such that adjustment of the position of the shaft along the piston of one pump to vary the stroke thereof, causes the shaft to move along the piston of the other pump and adjust its stroke. No provision is made in such system for adjustment of the volume of one material introduced into the system independently of the other material. In addition, no provision is made for the possibility that the pumps may cavitate. Thus, volumetric metering may not give accurate mass ratio of the two materials. If the liquid coating composition has a high viscosity, the piston chamber may not completely fill. Likewise, any pressure drop which the supercritical fluid goes through in entering the pump can cause it to vaporize and fill up a portion of the pump chamber with gas.
Another problem with the pumping arrangement in systems of the type disclosed in the Lee et al U.S. Pat. No. 4,923,720 is that piston pumps inherently produce instantaneous flow variations which, in turn, result in errors in the desired ratio of the liquid coating composition and supercritical fluid forming the coating material solution. These instantaneous flow variations occur when the piston of each piston pump reaches the end of its stroke and moves in the opposite direction, i.e., a momentary pause is produced as the piston reverses direction and the resulting output flow can be somewhat uneven or pulsed. As mentioned above, it is important to carefully control the mass ratio of supercritical fluid and liquid coating composition which form the coating material solution, and such flow variations caused by the piston pumps which supply the two components may lead to the production of a solution having less than the optimum ratio. Additionally, such flow variations can result in the addition of too much supercritical carbon dioxide to the solution and cause it to transition from single phase to two phase. For some types of liquid coating compositions, the presence of a second phase in the solution adversely affects the application characteristics of the solution, while in others the presence of a second phase is desirable.
One suggestion for more accurately controlling the supply of liquid coating composition and supercritical fluid to the system has been to employ metering gear pumps as a replacement for piston pumps. Metering gear pumps are highly accurate and produce a continuous, even output flow which can be controlled to the extent required to ensure that the coating material solution contains the proper ratio of liquid coating composition and supercritical fluid. The problem with metering gear pumps, however, is that they are susceptible to damage and wear by the polymeric and pigmentary solids contained in the liquid coating composition. It has been found that the gear teeth of metering gear pumps become relatively quickly worn by the solids content of the liquid coating composition, requiring variation in pump speed to achieve the same flow up to the point where the pumps become too worn out to operate and must be replaced.
Finally, systems of the type disclosed in the Lee U.S. Pat. No. 4,923,720 for metering a ratio of material into the application system assume that the supercritical fluid diluent is mixed evenly throughout the system and does not leak from the system. It has been found that the supercritical fluids, having a density significantly lower than the liquid coating compositions, tend to separate out into crevices within the system. Pressure and temperature changes, particularly during start-up and shut-down, can cause these trapped bubbles to flow back into the main loop, resulting in a non-steady dispensing pattern since the ratio between the liquid coating composition and supercritical fluid has been changed. In addition, it has been found that the flexible hoses required to convey the coating to the coating dispenser are typically constructed of a polymeric material that is relatively permeable to the supercritical fluid diluent. Thus, the diluent can escape from the system, resulting in an improper content of diluent which is nowhere detected or corrected for in the system. These problems result from the fact that the system has no ability to monitor the supercritical fluid content of the composition while it is in the loop.